[kingstnap]

DNA damage inevitably accumulates. The big reason children are younger than their parents DNA wise is because the parents' DNA undergo random recombination to create something that is the mixture of the two.

This doesn't help overall. Mixing two roughly equally broken things just yields the mean of the two. But the trick is that roughly 60 to 70% of conceptions will not survive to birth. This rejection sampling is ultimately what makes children younger.

If you had a population of single cells that didn't undergo this rejection sampling at some point, entropy and Muller's ratchet would actually age the entire population and kill it.

[lossolo]

You are right that DNA damage inevitably accumulates and that selection (including miscarriages) weeds out embryos with severe defects but that doesn’t fully explain why a newborn’s biological age is near zero.

What scientists usually mean by "cellular age" isn’t mutation load, it’s the epigenetic and functional state of cells. During gametogenesis and early embryonic development DNA undergoes extensive repair, telomere maintenance and global epigenetic reprogramming that wipes and rewrites methylation patterns. This resets the cellular "clock" even though some mutations are passed on.

So while mutation load drifts slightly each generation, the reason babies start biologically young is this large scale reprogramming. That’s also why researchers are trying to mimic this process in adult cells (Yamanaka factors etc) to reverse aspects of aging.